In 1 Cor. 15 Paul is addressing believers who didn't think there was a resurrection of the dead. IF there is no resurrection of the dead Paul explains that there is no point in believing, and that life itself is then simply vain, without purpose. Today we might consider such a mindset nihilism, a belief that life is meaningless i.e. vain.
Paul employs classic "if/then" reasoning in his preamble of how life works now and will go into the future in the last portion of the chapter:
1 Cor. 15:
12 Now if Christ be preached that he rose from the dead,
how say some among you that there is no resurrection of the dead?
13 But
if there be no resurrection of the dead,
then is Christ not risen:
14 And
if Christ be not risen,
then is our preaching vain, and your faith is also vain.
Even believers who don't believe OSAS, IF they stopped and thought about it for a minute, they actually DO believe "eternal life" regardless. They just believe that the "eternal life" that the believers they suppose abandoned Christ and therefore Christ abandons them is an eternal life of torture.
So such believe OSAS in a negative sense of the term.
OSAS unto eternal torture.
Right beliefs do not save anyone. Believers are raised, present tense, the moment they believe.
Romans 10:9
That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead,
thou shalt be saved.
And what happens at that instant?
Ephesians 2:
4 But God, who is rich in mercy, for his great love wherewith he loved us,
5
Even when we were dead in sins, hath quickened us together with Christ, (
by grace ye are saved; )
6
And hath raised us up together,
and made us sit together in heavenly places
in Christ Jesus:
We all have a very hard time seeing this because we, in our fleshly sin burdened selves can't see it. It is in fact HIDDEN at this present time from us. Col. 3:3
When something such as this is hidden, believers do lean to forget what they can't see.
But the above is really all that is involved. Faith is quite simple in this regards, not complex whatsoever.
Such belief is not based on right beliefs or right anything. People who believe have often never even picked up a Bible and read it. They hear "testimony" of witnesses to the fact that God in Christ loves them, expressed His sacrifice for their sins, not counting sins against us, forgiving us, and rose from the dead, that we who believe this will have eternal life.
And yes, people who believe this do forget. This doesn't mean Christ and what He did and will do won't happen. It will happen, regardless of our intentions.
So what does the anti OSAS crowd really believe?
They believe God does count sins against believers. This won't cut the mustard of scripture. They have a wrong belief. I personally think such ill headed people will be saved regardless of their ill intentions toward other fallen believers, even though they themselves have forgotten the effect of Christ's sacrifice.
2 Corinthians 5:
19 that God was reconciling the world to himself in Christ,
not counting people’s sins against them. And he has committed to us the message of reconciliation.
When we witness about Jesus, we are simply trying to reconcile them to a hard fact that won't change, which is the above.
The non-OSAS crowd does NOT believe 2 Cor. 5:19.
Whether they do nor not will prove to be irrelevant anyway.